Display Device and Manufacturing Method Thereof

ABSTRACT

A novel display device that is highly convenient or reliable is provided. The display device includes a first display element, a second display element, a first transistor, a second transistor, and a third transistor. The first display element includes a liquid crystal layer. The second display element includes a light-emitting layer. The first transistor has a function of selecting the first display element. The second transistor has a function of selecting the second display element. The third transistor has a function of controlling the driving of the second display element. The first transistor and the second transistor are formed over the same surface. The third transistor is formed above the first transistor and the second transistor and includes one of a source electrode and a drain electrode of the second transistor as a gate electrode.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a display device and a manufacturing method thereof.

Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. Furthermore, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specific examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, a method for driving any of them, and a method for manufacturing any of them.

2. Description of the Related Art

A liquid crystal display device in which a surface-emitting light source is provided as a backlight and combined with a transmissive liquid crystal display device in order to reduce power consumption and suppress a reduction in display quality is known (see Patent Document 1).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No. 2011-248351

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide a novel display device that is highly convenient or reliable.

Another object of one embodiment of the present invention is to provide a display device with low power consumption and high display quality. Another object of one embodiment of the present invention is to provide a novel display device.

Note that the descriptions of these objects do not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

One embodiment of the present invention is a display device including a first display element, a second display element, a first transistor, a second transistor, and a third transistor. The first display element includes a liquid crystal layer. The second display element includes a light-emitting layer. The first transistor has a function of selecting the first display element. The second transistor has a function of selecting the second display element. The third transistor has a function of controlling the driving of the second display element. The first transistor and the second transistor are formed over the same surface. The third transistor is formed above the first transistor and the second transistor and includes one of a source electrode and a drain electrode of the second transistor as a gate electrode.

Another embodiment of the present invention is a display device including a first display element, a second display element, a first transistor, a second transistor, a third transistor, and a capacitor. The first display element includes a first pixel electrode and a liquid crystal layer. The second display element includes a second pixel electrode and a light-emitting layer. The first transistor is electrically connected to the first pixel electrode. The second transistor is electrically connected to the second pixel electrode. The third transistor is electrically connected to the second display element. The capacitor includes a pair of electrodes. One of the pair of electrodes includes a capacitor electrode. The other of the pair of electrodes includes the first pixel electrode. The first transistor and the second transistor are formed over the same surface. The third transistor is formed above the first transistor and the second transistor and includes one of a source electrode and a drain electrode of the second transistor as a gate electrode.

In the above embodiment, the capacitor electrode is preferably arranged below one or both of the first transistor and the second transistor.

In the above embodiment, the first pixel electrode preferably has a function of reflecting light, and the second pixel electrode preferably has a function of transmitting light.

In the above embodiment, the first pixel electrode includes one or both of silver and aluminum. The second pixel electrode preferably includes one or more of indium, zinc, tin, and silicon.

In the above embodiment, the third transistor preferably includes a plurality of gate electrodes.

In the above embodiment, one or more of the first transistor, the second transistor, and the third transistor preferably include an oxide semiconductor in a semiconductor layer.

In the above embodiment, the light-emitting layer preferably has a function of emitting light toward the liquid crystal layer side.

Another embodiment of the present invention is a display module including a touch sensor and the display device with any one of the above structures. Another embodiment of the present invention is an electronic device including a battery and the display device with any one of the above structures or the display module.

One embodiment of the present invention can provide a novel display device that is highly convenient or reliable. According to one embodiment of the present invention, a display device with low power consumption and high display quality can be provided. According to one embodiment of the present invention, a novel display device can be provided.

Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not necessarily achieve all the effects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a top view and a cross-sectional view illustrating a pixel included in a display device.

FIG. 2 is a cross-sectional view illustrating a pixel included in a display device.

FIGS. 3A and 3B are a top view and a cross-sectional view illustrating a method for manufacturing a pixel included in a display device.

FIGS. 4A and 4B are a top view and a cross-sectional view illustrating a method for manufacturing a pixel included in a display device.

FIGS. 5A and 5B are a top view and a cross-sectional view illustrating a method for manufacturing a pixel included in a display device.

FIGS. 6A and 6B are a top view and a cross-sectional view illustrating a method for manufacturing a pixel included in a display device.

FIGS. 7A and 7B are a top view and a cross-sectional view illustrating a method for manufacturing a pixel included in a display device.

FIGS. 8A and 8B are a top view and a cross-sectional view illustrating a method for manufacturing a pixel included in a display device.

FIGS. 9A and 9B are a top view and a cross-sectional view illustrating a method for manufacturing a pixel included in a display device.

FIGS. 10A and 10B are a top view and a cross-sectional view illustrating a method for manufacturing a pixel included in a display device.

FIGS. 11A and 11B are a top view and a cross-sectional view illustrating a method for manufacturing a pixel included in a display device.

FIGS. 12A and 12B are a top view and a cross-sectional view illustrating a method for manufacturing a pixel included in a display device.

FIGS. 13A and 13B are a top view and a cross-sectional view illustrating a method for manufacturing a pixel included in a display device.

FIG. 14 is a schematic view illustrating a display region of a display element.

FIG. 15 is a block diagram illustrating a display device.

FIG. 16 is a circuit diagram illustrating a pixel.

FIGS. 17A and 17B are perspective views illustrating an example of a touch panel.

FIG. 18 is a cross-sectional view illustrating an example of a touch sensor.

FIG. 19 is a cross-sectional view illustrating an example of a touch panel.

FIGS. 20A and 20B are a block diagram and a timing chart of a touch sensor.

FIGS. 21A to 21C each illustrate an atomic ratio range of an oxide semiconductor film.

FIGS. 22A to 22C are band diagrams of stacked-layer structures of oxide semiconductor films.

FIG. 23 illustrates a display module.

FIGS. 24A to 24E each illustrate an electronic device.

FIGS. 25A to 25E are perspective views each illustrating a display device.

FIGS. 26A and 26B show display results of a display device of Example.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be hereinafter described with reference to drawings. Note that the embodiments can be implemented in many different modes. It will be readily appreciated by those skilled in the art that modes and details can be changed in various ways without departing from the spirit and scope of the present invention. Therefore, the present invention should not be interpreted as being limited to the description in the following embodiments.

In the drawings, the size, the layer thickness, or the region is exaggerated for clarity in some cases. Therefore, embodiments of the present invention are not limited to such a scale. Note that the drawings are schematic views showing ideal examples, and embodiments of the present invention are not limited to shapes or values shown in the drawings.

Ordinal numbers such as “first,” “second,” and “third” in this specification are used in order to avoid confusion among components, and the terms do not limit the components numerically.

In this specification, terms for describing arrangement, such as “over” and “under,” are used for convenience for describing the positional relation between components with reference to drawings. The positional relation between components is changed as appropriate in accordance with a direction in which each component is described. Thus, there is no limitation on terms used in this specification, and description can be made appropriately depending on the situation.

In this specification and the like, a transistor is an element having at least three terminals of a gate, a drain, and a source. The transistor includes a channel region between the drain (a drain terminal, a drain region, or a drain electrode) and the source (a source terminal, a source region, or a source electrode), and current can flow between the source and the drain through the channel region. Note that in this specification and the like, a channel region refers to a region through which current mainly flows.

Functions of a “source” and a “drain” are sometimes replaced with each other when a transistor of opposite polarity is used or when the direction of current flowing is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be replaced with each other in this specification and the like.

In this specification and the like, the term “electrically connected” includes the case where components are connected through an object having any electric function. There is no particular limitation on an “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object. Examples of an “object having any electric function” include a switching element such as a transistor, a resistor, an inductor, a capacitor, and an element with a variety of functions, as well as an electrode and a wiring.

In this specification and the like, the terms “film” and “layer” can be interchanged with each other. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. Also, the term “insulating film” can be changed into the term “insulating layer” in some cases.

Embodiment 1

In this embodiment, a display device of one embodiment of the present invention and the manufacturing method thereof are described with reference to FIG. 1A to FIG. 16.

<1-1. Structure of Display Device>

First, the structure of the display device is described with reference to FIG. 15. A display device 500 illustrated in FIG. 15 includes a pixel portion 502, and gate driver circuit portions 504 a and 504 b and a source driver circuit portion 506 which are placed outside the pixel portion 502.

[Pixel Portion]

The pixel portion 502 includes pixels 10(1, 1) to 10(X, Y) arranged in X rows (X is a natural number of 2 or more) and Y columns (Y is a natural number of 2 or more). Each of the pixels 10(1, 1) to 10(X, Y) includes two display elements having different functions. One of the two display elements has a function of reflecting incident light, and the other has a function of emitting light. Note that the details of the two display elements are described later.

[Gate Driver Circuit Portion]

Some or all of the gate driver circuit portions 504 a and 504 b and the source driver circuit portion 506 are preferably formed over a substrate over which the pixel portion 502 is formed. Thus, the number of components and the number of terminals can be reduced. In the case where some or all of the gate driver circuit portions 504 a and 504 b and the source driver circuit portion 506 are not formed over the substrate over which the pixel portion 502 is formed, a separately prepared driver circuit board (e.g., a driver circuit board formed using a single crystal semiconductor film or a polycrystalline semiconductor film) may be formed in the display device 500 by chip on glass (COG) or tape automated bonding (TAB).

The gate driver circuit portions 504 a and 504 b have a function of outputting a signal (a scan signal) for selecting the pixels 10(1, 1) to 10(X, Y). The source driver circuit portion 506 has a function of supplying a signal (data signal) for driving the display elements included in the pixels 10(1, 1) to 10(X, Y).

The gate driver circuit portion 504 a has a function of controlling the potentials of wirings supplied with scan signals (hereinafter, such wirings are referred to as a scan line GL_L[m], a scan line GL_L[m+1], and a scan line GL_L[X]) or a function of supplying an initialization signal. The gate driver circuit portion 504 b has a function of controlling the potentials of wirings supplied with scan signals (hereinafter, such wirings are referred to as a scan line GL_E1[m], a scan line GL_E1[m+1], a scan line GL_E2[m], a scan line GL_E2[m+1], a scan line GL_E1[X], and a scan line GL_E2[X]) or a function of supplying an initialization signal. In the above, m is a natural number less than or equal to X.

Without being limited to the above functions, the gate driver circuit portions 504 a and 504 b may each have a function of controlling or supplying another signal.

Although the structure in which the two gate driver circuit portions 504 a and 504 b are provided as gate driver circuit portions is illustrated in FIG. 15, the number of gate driver circuit portions is not limited thereto, and one or three or more gate driver circuit portions may be provided.

The source driver circuit portion 506 has a function of generating a data signal to be written to the pixels 10(1, 1) to 10(X, Y) which is based on an image signal, a function of controlling the potentials of wirings to which data signals are supplied (signal lines SL_L[n], SL_L[n+1], SL_L[Y], SL_E1[n], SL_E1[n+1], SL_E1[Y], SL_E2[n], SL_E2[n+1], and SL_E2[Y]), or a function of supplying an initialization signal. Note that in the above description, n is a natural number less than or equal to Y.

Without being limited to the above functions, the source driver circuit portion 506 may have a function of generating or supplying another signal.

The source driver circuit portion 506 includes a plurality of analog switches or the like. The source driver circuit portion 506 can output, as data signals, time-divided image signals obtained by sequentially turning on the plurality of analog switches.

Although the structure where one source driver circuit portion 506 is provided is illustrated in FIG. 15, the number of the source driver circuit portions is not limited thereto, and a plurality of source driver circuit portions may be provided in the display device 500. For example, two source driver circuit portions may be provided so that the signal lines SL_L[n], SL_L[n+1], and SL_L[Y] are controlled by one of the source driver circuit portions and the signal lines SL_E1[n], SL_E1[n+1], SL_E1[Y], SL_E2[n], SL_E2[n+1], and SL_E2[Y] are controlled by the other of the source driver circuit portions.

A pulse signal is input to each of the pixels 10(1, 1) to 10(X, Y) through one of the scan lines GL_L[m], GL_L[m+1], and GL_L[X]. A data signal is input to each of the pixels 10(1, 1) to 10(X, Y) through one of the signal lines SL_L[n], SL_L[n+1], SL_L[Y], SL_E1[n], SL_E1[n+1], SL_E1[Y], SL_E2[n], SL_E2[n+1], and SL_E2[Y].

For example, to the pixel 10(m, n) in the m-th row and the n-th column, a pulse signal is input from the gate driver circuit portion 504 a through the scan line GL_L[m], and a data signal is input from the source driver circuit portion 506 through the signal line SL_L[n] in accordance with the potential of the scan line GL_L[m].

For example, the pixel 10(m, n) in the m-th row and the n-th column is supplied with pulse signals from the gate driver circuit portion 504 b through the scan lines GL_E1[m] and GL_E2[m] and supplied with a data signal from the source driver circuit portion 506 through the signal lines SL_E1[n] and SL_E2[n] in accordance with the potentials of the scan lines GL_E1[m] and GL_E2[m].

The pixel 10(m, n) includes two display elements as described above. The scan lines GL_L[m], GL_L[m+1], and GL_L[X] are wirings which control the potential of one of the two display elements. The scan lines GL_E1[m], GL_E1[m+1], GL_E1[X], GL_E2[m], GL_E2[m+1], and GL_E2[X] are wirings which control the potential of the other of the two display elements.

The scan lines SL_L[n], SL_L[n+1], and SL_L[Y] are wirings which control the potential of one of the two display elements. The signal lines SL_E1[n], SL_E1[n+1], SL_E1[Y], SL_E2[n], SL_E2[n+1], and SL_E2[Y] are wirings which control the potential of the other of the two display elements.

[External Circuit]

An external circuit 508 is connected to the display device 500. Note that the display device 500 may include the external circuit 508.

As shown in FIG. 15, the external circuit 508 is electrically connected to wirings supplied with anode potentials (hereinafter referred to as wirings ANODE).

<1-2. Circuit Configuration of Pixels>

Next, the circuit configuration of the pixel 10(m, n) is described with reference to FIG. 16.

FIG. 16 is a circuit diagram showing the pixel 10(m, n) which is included in the display device 500 of one embodiment of the present invention.

The pixel 10(m, n) includes the scan lines GL_L[m], GL_E1[m], and GL_E2[m] and signal lines SL_E1[n], SL_L[n], and SL_E2[n]. The pixel 10(m, n) includes transistors MA1 to MA4, a transistor MA5, transistors MB1 to MB4, a capacitor Cs_L, a display element 12, and a display element 14. In FIG. 16, the display element 14 includes display elements 14B, 14G, 14R, and 14W. The display element 14B has a function of emitting blue light. The display element 14G has a function of emitting green light. The display element 14R has a function of emitting red light. The display element 14W has a function of emitting white light.

The pixel 10(m, n) includes a wiring TCOM which is electrically connected to the display element 12 and the display element 14, a wiring CSCOM which is electrically connected to the display element 12, and a wiring ANODE and a wiring CATHODE which are electrically connected to the display element 14.

Each of the scan line GL_L[m], the signal line SL_L[n], the wiring CSCOM, and the wiring TCOM is a wiring for driving the display element 12. Each of the scan line GL_E1[m], the scan line GL_E2[m], the signal line SL_E1[n], the signal line SL_E2[n], the wiring ANODE, the wiring CATHODE, and the wiring TCOM is a wiring for driving the display element 14.

<1-3. Structure Example of First Display Element>

The display element 12 has a function of controlling transmission or reflection of light. In particular, the display element 12 is preferably a reflective display element which controls reflection of light. The display element 12 serving as a reflective display element can reduce power consumption of the display device because display can be performed with the use of external light. For example, the display element 12 may have a combined structure of a reflective film, a liquid crystal element, and a polarizing plate, a structure using a micro electro mechanical systems (MEMS), or the like.

<1-4. Structure Example of Second Display Element>

The display element 14 has a function of emitting light. Therefore, the display element 14 may be rephrased as a light-emitting element. For example, an electroluminescence element (also referred to as an EL element), or a light-emitting diode may be used as the display element 14.

As described above, in the display device of one embodiment of the present invention, display elements with different functions like the display elements 12 and 14 are used. In the case where a reflective liquid crystal element is used as one of the display elements and a transmissive EL element is used as the other of the display elements, a novel display device that is highly convenient or reliable can be provided. Furthermore, a display device with low power consumption and high display quality can be provided when a reflective liquid crystal element is used in an environment with bright external light and a transmissive EL element is used in an environment with weak external light.

<1-5. Driving Method for Display Element>

Next, a driving method of the display element 12 and the display element 14 is described with reference to FIG. 16. Note that in the following description, a liquid crystal element is used as the display element 12, and a light-emitting element is used as the display element 14 (the display elements 14B, 14G, 14R, and 14W).

<Driving Method for First Display Element>

In the pixel 10(m, ii), a gate electrode of the transistor MA5 is electrically connected to the scan line GL_L[m]. One of a source electrode and a drain electrode of the transistor MA5 is electrically connected to the signal line SL_L[n], and the other is electrically connected to one of a pair of electrodes of the display element 12. The transistor MA5 has a function of controlling whether to write a data signal by switching the on and off states.

The other of the pair of electrodes of the display element 12 is electrically connected to the wiring TCOM.

One of a pair of electrodes of the capacitor Cs_L is electrically connected to the other of the source electrode and the drain electrode of the transistor MA5 and one of a pair of electrodes of the display element 12, and the other of the pair of electrodes of the capacitor Cs_L is electrically connected to the wiring CSCOM. The capacitor Cs_L has a function of storing data written to the pixel 10(m, n).

For example, the gate driver circuit portion 504 a in FIG. 15 selects the pixels 10(m, 1) to 10(m, Y) to turn on the transistors MA5, and data of data signals are written. When the transistor MA5 is turned off, the pixels 10(m, 1) to (m, Y) to which the data has been written is brought into a holding state. This operation is sequentially performed row by row; thus, an image can be displayed.

[Driving Method for Second Display Element]

In the pixel 10(m, n), a gate electrode of the transistor MA1 is electrically connected to the scan line GL_E1[m]. One of a source electrode and a drain electrode of the transistor MA1 is electrically connected to the signal line SL_E1[n] and the other of the source electrode and the drain electrode is electrically connected to a gate electrode of the transistor MB1 and the wiring TCOM. The transistor MA1 has a function of controlling whether to write a data signal by switching the on and off states.

One of a source electrode and a drain electrode of the transistor MB1 is electrically connected to one of a pair of electrodes of the display element 14B. The other of the source electrode and the drain electrode of the transistor MB1 is electrically connected to the wiring ANODE. The other of the pair of electrodes of the display element 14B is electrically connected to the wiring CATHODE. The transistor MB1 functions as what is called a driving transistor which controls current supplied to the display element 14B.

A capacitor is formed between the other of the source electrode and the drain electrode of the transistor MA1 and the wiring ANODE. The capacitor has a function of storing data written to the pixel 10(m, n). The transistor MB1 includes a back gate electrode. The back gate electrode of the transistor MB1 is electrically connected to one of the source electrode and the drain electrode thereof.

In the pixel 10(m, n), a gate electrode of the transistor MA2 is electrically connected to the scan line GL_E1[m]. One of a source electrode and a drain electrode of the transistor MA2 is electrically connected to the signal line SL_E2[n] and the other of the source electrode and the drain electrode is electrically connected to a gate electrode of the transistor MB2 and the wiring TCOM. The transistor MA2 has a function of controlling whether to write a data signal by switching the on and off states.

One of a source electrode and a drain electrode of the transistor MB2 is electrically connected to one of a pair of electrodes of the display element 14G. The other of the source electrode and the drain electrode of the transistor MB2 is electrically connected to the wiring ANODE. The transistor MB2 functions as a so-called driving transistor which controls current supplied to the display element 14G.

A capacitor is formed between the other of the source electrode and the drain electrode of the transistor MA2 and the wiring ANODE. The capacitor has a function of storing data written to the pixel 10(m, n). The transistor MB2 includes a back gate electrode. The back gate electrode is electrically connected to one of the source electrode and the drain electrode of the transistor MB2.

In the pixel 10(m, n), a gate electrode of the transistor MA3 is electrically connected to the scan line GL_E2[m]. One of a source electrode and a drain electrode of the transistor MA3 is electrically connected to the signal line SL_E1[n] and the other of the source electrode and the drain electrode is electrically connected to a gate electrode of the transistor MB3 and the wiring TCOM. The transistor MA3 has a function of controlling whether to write a data signal by switching the on and off states.

One of a source electrode and a drain electrode of the transistor MB3 is electrically connected to one of a pair of electrodes of the display element 14R. The other of the source electrode and the drain electrode of the transistor MB3 is electrically connected to the wiring ANODE. The transistor MB3 functions as a so-called driving transistor which controls current supplied to the display element 14R.

A capacitor is formed between the other of the source electrode and the drain electrode of the transistor MA3 and the wiring ANODE. The capacitor has a function of storing data written to the pixel 10(m, n). The transistor MB3 includes a back gate electrode. The back gate electrode is electrically connected to one of the source electrode and the drain electrode of the transistor MB3.

In the pixel 10(m, n), a gate electrode of the transistor MA4 is electrically connected to the scan line GL_E2[m]. One of a source electrode and a drain electrode of the transistor MA4 is electrically connected to the signal line SL_E2 [n] and the other of the source electrode and the drain electrode is electrically connected to a gate electrode of the transistor MB4 and the wiring TCOM. The transistor MA4 has a function of controlling whether to write a data signal by switching the on and off states.

One of a source electrode and a drain electrode of the transistor MB4 is electrically connected to one of a pair of electrodes of the display element 14W. The other of the source electrode and the drain electrode of the transistor MB4 is electrically connected to the wiring ANODE. The transistor MB4 functions as a so-called driving transistor which controls current supplied to the display element 14W.

A capacitor is formed between the other of the source electrode and the drain electrode of the transistor MA4 and the wiring ANODE. The capacitor has a function of storing data written to the pixel 10(m, n). The transistor MB3 includes a back gate electrode. The back gate electrode is electrically connected to one of the source electrode and the drain electrode of the transistor MB4.

For example, the gate driver circuit portion 504 b in FIG. 15 selects the pixels 10(m, 1) to 10(m, Y) to turn on the transistors MA1 to MA4, and data of data signals are written. When the transistors MA1 to MA4 are turned off, the pixels 10(m, 1) to 10(m, Y) to which the data has been written are brought into a holding state. Furthermore, the amount of current flowing between the source electrode and the drain electrode of the transistor MB1 is controlled in accordance with the potential of the written data signal. The display element 14 emits light with a luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image can be displayed.

In this manner, two display elements can be controlled separately with the use of different transistors in the display device of one embodiment of the present invention. Accordingly, a display device having high display quality can be provided.

As illustrated in FIG. 16, the driving transistors of the display element 14 (the transistors MB1 to MB4) each include a back gate electrode, i.e., each of the transistors includes a plurality of gate electrodes; thus, the reliability or driving capability of the transistor can be improved. For example, as illustrated in FIG. 16, the back gate electrode is connected to one of the source electrode and the drain electrode, whereby a potential of the transistor on the back channel side can be fixed. Although not shown in the drawing, when a back gate electrode is connected to a gate electrode (a first gate electrode or a front gate electrode), the current drive capability of the transistor can be increased.

Each of the transistors (the transistors MA1 to MA5 and the transistors MB1 to MB4) used in the display device of one embodiment of the present invention preferably includes an oxide semiconductor film. The transistor including an oxide semiconductor film can have relatively high field-effect mobility and thus can operate at high speed. The off-state current of the transistor including an oxide semiconductor film is extremely low. Therefore, the luminance of the display device can be kept even when the refresh rate of the display device is lowered, so that power consumption can be lowered.

A progressive type display, an interlace type display, or the like can be employed as the display type of the display element 12 and the display element 14. Further, color components controlled in a pixel at the time of color display are not limited to blue, green, red, or white. For example, one or more colors of yellow, cyan, magenta, and the like may be added to blue, green, red, or white. Further, the size of a display region may be different in each dot of a color component. However, the display device of one embodiment of the present invention is not limited to a color display device and can be applied to a monochrome display device.

Note that the display device 500 can perform grayscale display using at least one of the display elements 12 and 14. For example, since the display element 12 is what is called a reflective liquid crystal element, visibility can be increased under strong external light. In the case where grayscale display is performed using the display element 12, backlight or the like is not necessarily adjusted as compared to the case where a transmissive liquid crystal display device is used; thus, power consumption can be reduced.

On the other hand, since the display element 14 is what is called a light-emitting element, visibility can be improved under weak external light. In the case where grayscale display is performed using the display element 14, image quality such as contrast can be improved as compared to the case where a transmissive liquid crystal display device is used because light emission can be controlled in each pixel without using a backlight or the like.

The display device 500 may perform grayscale display using both the display elements 12 and 14. When grayscale display is performed using both the display elements 12 and 14, visibility can be improved as compared to the case where grayscale display is performed using one of the display elements 12 and 14.

<1-6. Display Region of Display Element>

Here, display regions of the display elements 12 and 14 in the pixel 10(m, n) are described with reference to FIG. 14.

FIG. 14 is a schematic view illustrating the display regions of the pixel 10(m, n).

The display regions of the pixel 10(m, n) illustrated in FIG. 14 include a display region 12 d which functions as a display region of the display element 12, a display region 14Bd which functions as a display region of the display element 14B, a display region 14Gd which functions as a display region of the display element 14G, a display region 14Rd which functions as a display region of the display element 14R, and a display region 14Wd which functions as a display region of the display element 14W.

The display region 12 d includes a region which reflects light. The display region 14Bd includes a region which transmits blue light. The display region 14Gd includes a region which transmits green light. The display region 14Rd includes a region which transmits red light. The display region 14Wd includes a region which transmits white light.

For example, when the display regions of the pixel 10(m, n) are provided as illustrated in FIG. 14, a monochrome image can be displayed on the display region 12 d and a full-color image can be displayed by the display regions 14Gd, 14Rd, and 14Wd.

<1-7. Structure Example of Display Device (Cross-Section)>

Next, an example of a cross-sectional structure of a pixel 10 included in the display device 500 is described with reference to FIGS. 1A and 1B.

FIG. 1A is an example of a top view of the pixel 10. FIG. 1B is a cross-sectional view corresponding to cross sections taken along dashed-dotted lines A1-A2, A3-A4, and A5-A6. Note that in the top view of the pixel 10 illustrated in FIG. 1A, some components are not illustrated in order to avoid complexity of the drawing.

In FIG. 1A, display regions adjacent to each other in the pixel (the regions corresponding to the display regions 14Rd, 14Gd, 14Bd, and 14Wd) are clearly shown. The same applies to the following top views of the pixel 10.

In the pixel 10 illustrated in FIGS. 1A and 1B, the display elements 12 and 14 and transistors Tr1, Tr2, and Tr3 are provided between substrates 80 and 90.

Note that the transistor Tr1 corresponds to the transistor MA5. The transistor Tr2 corresponds to any one of the transistors MA1 to MA4. The transistor Tr3 corresponds to any one of the transistors MB1 to MB4.

The display element 12 includes a liquid crystal layer 96. The display element 14 includes an EL layer 76. The transistor Tr1 has a function of selecting the display element 12. The transistor Tr2 has a function of selecting the display element 14. The transistor Tr3 has a function of controlling driving of the display element 14. The transistors Tr1 and Tr2 are formed over the same surface. The transistor Tr3 is formed above the transistors Tr1 and Tr2 and includes any one of the source electrode and the drain electrode of the transistor Tr2 as a gate electrode.

Note that the display element 12 includes a conductive film 36 functioning as a first pixel electrode. The transistor Tr1 is electrically connected to the conductive film 36 and has a function of selecting the display element 12. The transistor Tr3 is electrically connected to a conductive film 70 and has a function of selecting the display element 14.

The pixel 10 includes a capacitor 16. The capacitor 16 includes a pair of electrodes. One of the pair of electrodes includes a conductive film 42 functioning as a capacitor electrode. The other of the pair of electrodes includes the conductive film 36. Note that the conductive film 42 is provided below one or both of the transistors Tr1 and Tr2.

The capacitor 16 corresponds to the capacitor Cs_L described above.

When the conductive film 42 functioning as a capacitor electrode is positioned below one or both of the transistors Tr1 and Tr2, noise due to data rewriting of the display element 12, i.e., noise due to data rewriting of the liquid crystal element in the pixel can be reduced.

As illustrated in FIGS. 1A and 1B, the transistors Tr1 and Tr2 are formed over the same surface and the transistor Tr3 is formed above the transistors Tr1 and Tr2, whereby a circuit area can be reduced. Since the transistor Tr3 includes one of the source electrode and the drain electrode included in the transistor Tr2 as a gate electrode, a manufacturing process can be shortened.

As illustrated in FIGS. 1A and 1B, the transistors Tr1, Tr2, and Tr3 are preferably transistors each having an inverted staggered structure (also referred to as a bottom-gate structure). The transistor having a bottom gate structure can be manufactured in a relatively easy process. However, one embodiment of the present invention is not limited to this, and transistors each having a top gate structure may be used.

The display element 12 has a function of reflecting incident light. Note that the display element 12 is what is called a liquid crystal element and includes the liquid crystal layer 96 between a pair of electrodes. One of the pair of electrodes includes the conductive film 36, and the other of the pair of electrodes includes a conductive film 92. As illustrated in FIGS. 1A and 1B, the display element 12 may include alignment films 94 and 98 in contact with the liquid crystal layer 96. The conductive film 36 functions as a reflective electrode. When light which enters from the outside is reflected by the conductive film 36 as shown by a dashed arrow in FIG. 1B, the light can be reflected to the viewing side.

The display element 14 has a function of emitting light. Note that the display element 14 is what is called a light-emitting element and includes the EL layer 76 between a pair of electrodes. One of the pair of electrodes includes the conductive film 70, and the other of the pair of electrodes includes a conductive film 78. The conductive film 78 functions as a reflective electrode. As shown by a dashed-two dotted arrow in FIG. 1B, light emitted from the EL layer 76 is reflected by the conductive film 78, passes through the conductive film 70, and is extracted to the liquid crystal layer 96 side. Light emitted from the display element 14 is extracted through an opening provided in the conductive film 36 to the substrate 90 side. In FIG. 1B, the opening is shown as a display region 14 d.

The display element 14 may be a light-emitting element in which the EL layer 76 emitting light of red (R), green (G), blue (B), and the like is formed using a fine metal mask (FMM). Note that the structure of the display element 14 is not limited thereto, and the display element 14 may be an element emitting white (W) light and the light from the element may pass through coloring films to obtain light of R, G, B, and the like.

An example of the above structure is shown in FIG. 2. FIG. 2 is a cross-sectional view of a modification example of the pixel 10 illustrated in FIG. 1B. As illustrated in FIG. 2, light from the display element 14 is extracted to the outside through a coloring film 69. As illustrated in FIG. 2, the coloring film 69 is preferably provided to cover part of the transistor Tr1. In particular, when a channel region of the transistor Tr1 is covered with the coloring film 69, the amount of light which enters the channel region can be reduced. Reducing the amount of light which enters the channel region can increase light resistance of the transistor Tr1.

<1-8. Manufacturing Method of Display Device>

Next, a manufacturing method of the pixel 10 included in the display device 500 illustrated in FIGS. 1A and 1B is described with reference to FIG. 3A to FIG. 13B.

First, a conductive film 31, an insulating film 32, and an insulating film 34 are formed in this order over a substrate 30. Then, a conductive film is formed over the insulating film 34 and processed into an island shape, whereby the conductive film 36 is formed (see FIGS. 3A and 3B).

The conductive film 31 functions as a first separation layer. The insulating films 32 and 34 function as a second separation layer. The conductive film 36 functions as a reflective film. Note that the conductive film 36 may be called a first pixel electrode. The first pixel electrode preferably contains one or both of silver and aluminum. The first pixel electrode containing silver and aluminum can increase its reflectance. Note that examples of the first pixel electrode containing silver can include an alloy containing silver, palladium, and copper.

Next, a conductive film is formed over the insulating film 34 and the conductive film 36 and processed into an island shape, whereby a conductive film 38 is formed. Then, an insulating film 40 is formed over the conductive films 36 and 38 (see FIGS. 4A and 4B).

The conductive film 38 is formed using a light-transmitting conductive material and functions as part of one of the pair of electrodes of the display element 12. Note that the conductive film 38 has the following function. The conductive film 38 has a function of controlling the alignment of the liquid crystal layer 96 included in the display element 12. For example, in the case where the conductive film 38 is not provided, the alignment state of the liquid crystal layer 96 overlapping with the opening of the conductive film 36 (corresponding to the display region 14 d shown in FIG. 1B) cannot be controlled in some cases. In contrast, in the case where the conductive film 38 is provided, the alignment state of the liquid crystal layer 96 can be appropriately controlled by a potential applied to the conductive films 36 and 38.

Next, a conductive film is formed over the insulating film 40 and processed into an island shape, whereby the conductive film 42 is formed. Next, an insulating film 44 is formed over the conductive film 42. Then, a conductive film is formed over the insulating film 44 and processed into an island shape, whereby conductive films 46 a, 46 b, and 46 c are formed. Then, an insulating film 48 is formed over the insulating film 44, the conductive film 46 a, the conductive film 46 b, and the conductive film 46 c. Then, an oxide semiconductor film is formed over the insulating film 48 and processed into an island shape, whereby oxide semiconductor films 50 a and 50 b are formed. Then, an opening 51 is formed in a desired region of the insulating film 40, the insulating film 44, and the insulating film 48. Note that the opening 51 is formed so as to expose part of the conductive film 36 (see FIGS. 5A and 5B).

The conductive film 42 functions as the other of the pair of electrodes of the capacitor 16. That is, the capacitor 16 includes the conductive film 36, the conductive film 38, the insulating film 40, and the conductive film 42. The conductive films 36 and 38 function as one of the pair of electrodes of the capacitor 16. The conductive film 42 functions as the other of the pair of electrodes of the capacitor 16. The insulating film 40 functions as a dielectric film of the capacitor 16.

The conductive film 46 a functions as a gate electrode of the transistor Tr1. The conductive film 46 c functions as a gate electrode of the transistor Tr2.

Next, a conductive film is formed over the insulating film 48, the oxide semiconductor film 50 a, the oxide semiconductor film 50 b, and the opening 51 and processed into island shapes, whereby conductive films 52 a, 52 b, 52 c, 52 d, and 52 e are formed (see FIGS. 6A and 6B).

The conductive film 52 a functions as a source electrode or a drain electrode of the transistor Tr1. The conductive film 52 b functions as a source electrode or a drain electrode of the transistor Tr1. The conductive film 52 c functions as a gate electrode of the transistor Tr3. The conductive film 52 d functions as a source electrode or a drain electrode of the transistor Tr2. The conductive film 52 e functions as a source electrode or a drain electrode of the transistor Tr2.

Next, an insulating film 54 is formed over the insulating film 48, the oxide semiconductor films 50 a and 50 b, and the conductive films 52 a, 52 b, 52 c, 52 d, and 52 e. Then, an oxide semiconductor film is formed over the insulating film 54 and processed, whereby an island-shaped oxide semiconductor film 56 is formed. After that, a conductive film is formed over the island-shaped oxide semiconductor film 56 and processed into island shapes, whereby conductive films 58 a and 58 b are formed (see FIGS. 7A and 7B).

The conductive films 58 a and 58 b function as a source electrode and a drain electrode of the transistor Tr3.

Next, insulating films 60 and 62 are formed over the insulating film 54 and the conductive films 58 a and 58 b. Then, an opening 63 reaching the conductive film 58 b is formed in a desired region of the insulating films 60 and 62. Then, a conductive film is formed over the insulating film 62 so as to fill the opening 63 and processed into an island shape, whereby the conductive film 64 is formed (see FIGS. 8A and 8B).

The conductive film 64 functions as a back gate electrode of the transistor Tr3. The conductive film 64 is electrically connected to the conductive film 58 b.

Next, an insulating film 68 having an opening is formed over the insulating film 62 and the conductive film 64. Next, a conductive film is formed over the insulating film 68 and processed into an island shape, whereby the conductive film 70 is formed. Then, an insulating film 72 having an opening is formed over the insulating film 68 and the conductive film 70 (see FIGS. 9A and 9B).

The insulating film 68 functions as a planarization insulating film. The conductive film 70 functions as one of a pair of electrodes of the display element 14 and a second pixel electrode. Note that the conductive film 70 is electrically connected to the conductive film 58 b and the conductive film 64. The insulating film 72 has a function of separating part of the display element 14 from the other part between adjacent pixels. The conductive film 70 functioning as the second pixel electrode preferably contains one or more elements selected from indium, zinc, tin, and silicon. For example, as the conductive film 70, a conductive film containing indium, tin, and silicon can be used. In FIG. 9A, hatching of the conductive film 70 is seen transparently so that components under the conductive film 70 are clearly shown.

Next, a structure body 74 is formed over the insulating film 72. Then, the EL layer 76 and the conductive film 78 are formed over the insulating film 72 and the structure body 74 (see FIGS. 10A and 10B).

The structure body 74 has a function of making a predetermined gap between the display element 14 and the substrate 80. The EL layer 76 has a function of emitting light. The conductive film 78 functions as the other of the pair of electrodes of the display element 14.

Next, a sealing material 82 is applied on the conductive film 78, and the substrate 30 on which the transistor, the display element, and the like are formed and the substrate 80 are bonded to each other (see FIGS. 11A and 11B).

Then, the substrate 30 and the substrate 80 are separated from each other. In this embodiment, the substrate 30 and the substrate 80 are separated from each other at the vicinity of an interface between the conductive film 31 and the insulating film 32 (see FIGS. 12A and 12B).

When the element is separated at the interface between the element and the conductive film 31, a polar solvent (typically water), a nonpolar solvent, or the like is preferably added to the interface between the conductive film 31 and the insulating film 32. For example, it is preferable to use water in separating the element at the interface between the element and the conductive film 31 because damage caused by electrification in separation can be reduced.

As the conductive film 31, any of the following materials can be used. The conductive film 31 can have a single-layer structure or a stacked-layer structure containing an element selected from tungsten, molybdenum, titanium, tantalum, zinc, ruthenium, rhodium, palladium, osmium, iridium, gallium, and silicon; an alloy material containing any of the elements; or a compound material containing any of the elements. In the case of a layer containing silicon, a crystal structure of the layer containing silicon may be amorphous, microcrystal, polycrystal, or single crystal.

When the conductive film 31 is formed as a stacked-layer structure including a layer containing tungsten and a layer containing an oxide of tungsten, the layer containing tungsten may be formed and an insulating layer containing an oxide may be formed thereover so that the layer containing an oxide of tungsten is formed at the interface between the tungsten layer and the insulating layer. Alternatively, the layer containing an oxide of tungsten may be formed by performing thermal oxidation treatment, oxygen plasma treatment, dinitrogen monoxide (N₂O) plasma treatment, treatment with a highly oxidizing solution such as ozone water, or the like on the surface of the layer containing tungsten. Plasma treatment or heat treatment may be performed in an atmosphere of oxygen, nitrogen, or dinitrogen monoxide alone, or a mixed gas of any of these gasses and another gas. Surface condition of the conductive film 31 is changed by the plasma treatment or heat treatment, whereby adhesion between the conductive film 31 and the insulating film 32 formed later can be controlled.

Although the structure where the conductive film 31 is provided is described in this embodiment, the present invention is not limited thereto. For example, a structure where the conductive film 31 is not provided may be employed. In that case, an organic resin film may be formed in a region in which the conductive film 31 is formed. As the organic resin film, for example, a polyimide-based resin film, a polyamide-based resin film, an acrylic-based resin film, an epoxy-based resin film, or a phenol-based resin film can be used. Note that in the case where a polyimide-based resin film is used, a photosensitive and thermosetting organic resin material is preferably used. When a photosensitive and thermosetting organic resin material is used, the organic resin material can have a shape or the like.

In the case where the organic resin film is used instead of the conductive film 31, as a method for separating the element formed over the substrate 30, laser light (e.g., excimer laser having a wavelength of 308 nm or UV laser having a wavelength of 355 nm which is the third harmonic of a YAG laser) is irradiated from the lower side of the substrate 30 to weaken the organic resin film, whereby separation is conducted at an interface between the substrate 30 and the organic resin film, inside the organic resin film, or at an interface between the organic resin film and the insulating film 32.

In the case where irradiation with laser light is performed, a region having strong adhesion and a region having weak adhesion are formed between the substrate 30 and the insulating film 32 by adjustment of the irradiation energy density of the laser light, and then, the element may be separated from the substrate 30. As the laser light, linear laser light may be used.

Next, the insulating films 32 and 34 formed below the substrate 80 are removed, and the rear surface of the conductive films 36 and 38 is exposed (see FIGS. 13A and 13B).

The insulating films 32 and 34 can be removed by a dry etching method and/or a wet etching method.

Next, an alignment film 98 is formed in contact with the conductive films 36 and 38. Then, the substrate 90 over which the conductive film 92 and the alignment film 94 are formed is prepared, and a space between a side of the substrate 80 over which the alignment film 98 is formed and a side of the substrate 90 over which the alignment film 94 is formed is filled with the liquid crystal layer 96, whereby the display device 500 including the pixels 10 shown in FIGS. 1A and 1B can be manufactured.

<1-9. Components of Display Device>

Next, the components of the display device 500 and the manufacturing method thereof illustrated in FIG. 1A to FIG. 13B, FIG. 15, and FIG. 16 are described below.

[Substrate]

The substrates 30, 80, and 90 can be formed using a material having heat resistance high enough to withstand heat treatment in the manufacturing process.

Specifically, non-alkali glass, soda-lime glass, alkali glass, crystal glass, quartz, sapphire, or the like can be used. Alternatively, an inorganic insulating film may be used. Examples of the inorganic insulating film include a silicon oxide film, a silicon nitride film, a silicon oxynitride film, and an aluminum oxide film.

The non-alkali glass may have a thickness of greater than or equal to 0.2 mm and less than or equal to 0.7 mm, for example. The non-alkali glass may be polished to obtain the above thickness.

As the non-alkali glass, a large-area glass substrate having any of the following sizes can be used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, a large-sized display device can be manufactured.

Alternatively, as each of the substrates 30, 80, and 90, a single-crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon or silicon carbide, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like may be used.

For the substrates 30, 80, and 90, an inorganic material such as a metal may be used. Examples of the inorganic material such as a metal include stainless steel and aluminum.

Alternatively, for the substrates 30, 80, and 90, an organic material such as a resin, a resin film, or plastic may be used. Examples of the resin film include polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, polyurethane, an acrylic resin, an epoxy resin, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), and a resin having a siloxane bond.

For the substrates 30, 80, and 90, a composite material of an inorganic material and an organic material may be used. Examples of the composite material include a resin film to which a metal plate or a thin glass plate is bonded, a resin film into which a fibrous or particulate metal or a fibrous or particulate glass is dispersed, and an inorganic material into which a fibrous or particulate resin is dispersed.

Each of the substrates 30, 80, and 90 can at least support films or layers formed thereover or thereunder and may be one or more of an insulating film, a semiconductor film, and a conductive film.

[Conductive Film]

As each of the conductive films 31, 36, 38, 42, 46 a, 46 b, 46 c, 52 a, 52 b, 52 c, 52 d, 52 e, 58 a, 58 b, 64, 70, 78, and 92, a metal film having conductivity, a conductive film having a function of reflecting visible light, or a conductive film having a function of transmitting visible light may be used.

A material containing a metal element selected from aluminum, gold, platinum, silver, copper, chromium, tantalum, titanium, molybdenum, tungsten, nickel, iron, cobalt, palladium, and manganese can be used for the metal film having conductivity. Alternatively, an alloy containing any of the above metal elements may be used.

For the metal film having conductivity, specifically a two-layer structure in which a copper film is stacked over a titanium film, a two-layer structure in which a copper film is stacked over a titanium nitride film, a two-layer structure in which a copper film is stacked over a tantalum nitride film, a three-layer structure in which a titanium film, a copper film, and a titanium film are stacked in this order, or the like may be used. In particular, a conductive film containing a copper element is preferably used because the resistance can be reduced. As an example of the conductive film containing a copper element, an alloy film containing copper and manganese is given. The alloy film is favorable because it can be processed by a wet etching method.

As the metal film having conductivity, a conductive macromolecule or a conductive polymer may be used.

For the conductive film having a function of reflecting visible light, a material containing a metal element selected from gold, silver, copper, and palladium can be used. In particular, a conductive film containing a silver element is preferably used because reflectance of visible light can be improved.

For the conductive film having a function of transmitting visible light, a material containing an element selected from indium, tin, zinc, gallium, and silicon can be used. Specifically, an In oxide, a Zn oxide, an In—Sn oxide (also referred to as ITO), an In—Sn—Si oxide (also referred to as ITSO), an In—Zn oxide, an In—Ga—Zn oxide, or the like can be used.

As the conductive film having a function of transmitting visible light, a film containing graphene or graphite may be used. The film containing graphene can be formed in the following manner: a film containing graphene oxide is formed and is reduced. As a reducing method, a method with application of heat, a method using a reducing agent, or the like can be employed.

The conductive films 31, 36, 38, 42, 46 a, 46 b, 46 c, 52 a, 52 b, 52 c, 52 d, 52 e, 58 a, 58 b, 64, 70, 78, and 92 can be formed by electroless plating. As a material that can be deposited by electroless plating, for example, one or more elements selected from Cu, Ni, Al, Au, Sn, Co, Ag, and Pd can be used. It is further favorable to use Cu or Ag because the resistance of the conductive film can be reduced.

When the conductive film is formed by electroless plating, a diffusion prevention film may be formed under the conductive film to prevent component elements of the conductive film from diffusing outward. A seed film that can make the conductive film grow may be formed between the diffusion prevention film and the conductive film. The diffusion prevention film can be formed by sputtering, for example. For the diffusion prevention film, a tantalum nitride film or a titanium nitride film can be used, for example. The seed film can be formed by electroless plating. For the seed film, a material similar to the material for the conductive film that can be formed by electroless plating can be used.

Alternatively, each of the conductive films can be a conductive film having a function of reflecting visible light and a function of transmitting visible light by combining a conductive film having a function of reflecting visible light and a conductive film having a function of transmitting visible light. For example, one of the pair of electrodes included in the display element 14 is a conductive film having a function of reflecting visible light and the other of the pair of electrodes is a conductive film having a function of reflecting visible light and transmitting visible light. The above structure can be a micro optical resonator (microcavity) structure utilizing a resonant effect of light between a pair of electrodes, so that the intensity of light having a specific wavelength can be increased.

Note that the other of the pair of electrodes included in the display element 14 (e.g., the conductive film 70) can have a stacked-layer structure including an In—Sn—Si oxide and an alloy containing silver. The alloy containing silver may be a thin film (e.g., the thickness is less than or equal to 50 nm, preferably less than or equal to 30 nm) so that visible light can be transmitted.

[Insulating Film]

For the insulating films 32, 34, 40, 44, 48, 54, 60, 62, 68, and 72, an inorganic insulating material, an organic insulating material, or an insulating composite material including an insulating inorganic material and an insulating organic material can be used.

Examples of the insulating inorganic material include a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, and an aluminum oxide film. Alternatively, a plurality of the above inorganic materials may be stacked.

Examples of the above organic insulating material include materials that include polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, polyurethane, an acrylic-based resin, an epoxy-based resin, and a resin having a siloxane bond. As the insulating organic material, a photosensitive material may be used.

[Oxide Semiconductor Film]

The oxide semiconductor films 50 a, 50 b, and 56 are formed using a metal oxide such as an In-M-Zn oxide (M is Al, Ga, Y, or Sn). Alternatively, an In—Ga oxide or an In—Zn oxide may be used for the oxide semiconductor films 50 a, 50 b, and 56. Note that an oxide semiconductor film that can be used as the oxide semiconductor films 50 a, 50 b, and 56 will be described in detail in Embodiment 3.

[Liquid Crystal Layer]

As examples of the liquid crystal layer 96, thermotropic liquid crystal, low-molecular liquid crystal, high-molecular liquid crystal, polymer dispersed liquid crystal, ferroelectric liquid crystal, and anti-ferroelectric liquid crystal are given. Alternatively, a liquid crystal material which exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like may be used. Furthermore, a liquid crystal material exhibiting a blue phase may be used.

For a driving method of the liquid crystal layer 96, an in-plane switching (IPS) mode, a twisted nematic (TN) mode, an FFS mode, an axially symmetric aligned micro-cell (ASM) mode, an optically compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the like can be used. In addition, the liquid crystal layer 96 can be driven by, for example, a vertical alignment (VA) mode such as a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an electrically controlled birefringence (ECB) mode, a continuous pinwheel alignment (CPA) mode, or an advanced super view (ASV) mode can be used.

[EL Layer]

The EL layer 76 includes at least a light-emitting material. Examples of the light-emitting material include an organic compound and an inorganic compound such as a quantum dot.

The organic compound and the inorganic compound can be formed by an evaporation method (including a vacuum evaporation method), an ink-jet method, a coating method, or gravure printing, for example.

Examples of materials that can be used for the organic compound include a fluorescent material and a phosphorescent material. A fluorescent material is preferably used in terms of the lifetime, while a phosphorescent material is preferably used in terms of the efficiency. Furthermore, both of a fluorescent material and a phosphorescent material may be used.

A quantum dot is a semiconductor nanocrystal with a size of several nanometers and contains approximately 1×10³ to 1×10⁶ atoms. Since energy shift of quantum dots depend on their size, quantum dots made of the same substance emit light with different wavelengths depending on their size; thus, emission wavelengths can be easily adjusted by changing the size of quantum dots.

Since a quantum dot has an emission spectrum with a narrow peak, emission with high color purity can be obtained. In addition, a quantum dot is said to have a theoretical internal quantum efficiency of approximately 100%, which far exceeds that of a fluorescent organic compound, i.e., 25%, and is comparable to that of a phosphorescent organic compound. Therefore, a quantum dot can be used as a light-emitting material to obtain a light-emitting element having high emission efficiency. Furthermore, since a quantum dot which is an inorganic compound has high inherent stability, a light-emitting element which is favorable also in terms of lifetime can be obtained.

Examples of a material of a quantum dot include a Group 14 element in the periodic table, a Group 15 element in the periodic table, a Group 16 element in the periodic table, a compound of a plurality of Group 14 elements in the periodic table, a compound of an element belonging to any of Groups 4 to 14 in the periodic table and a Group 16 element in the periodic table, a compound of a Group 2 element in the periodic table and a Group 16 element in the periodic table, a compound of a Group 13 element in the periodic table and a Group 15 element in the periodic table, a compound of a Group 13 element in the periodic table and a Group 17 element in the periodic table, a compound of a Group 14 element in the periodic table and a Group 15 element in the periodic table, a compound of a Group 11 element in the periodic table and a Group 17 element in the periodic table, iron oxides, titanium oxides, spinel chalcogenides, and semiconductor clusters.

Specific examples include, but are not limited to, cadmium selenide; cadmium sulfide; cadmium telluride; zinc selenide; zinc oxide; zinc sulfide; zinc telluride; mercury sulfide; mercury selenide; mercury telluride; indium arsenide; indium phosphide; gallium arsenide; gallium phosphide; indium nitride; gallium nitride; indium antimonide; gallium antimonide; aluminum phosphide; aluminum arsenide; aluminum antimonide; lead selenide; lead telluride; lead sulfide; indium selenide; indium telluride; indium sulfide; gallium selenide; arsenic sulfide; arsenic selenide; arsenic telluride; antimony sulfide; antimony selenide; antimony telluride; bismuth sulfide; bismuth selenide; bismuth telluride; silicon; silicon carbide; germanium; tin; selenium; tellurium; boron; carbon; phosphorus; boron nitride; boron phosphide; boron arsenide; aluminum nitride; aluminum sulfide; barium sulfide; barium selenide; barium telluride; calcium sulfide; calcium selenide; calcium telluride; beryllium sulfide; beryllium selenide; beryllium telluride; magnesium sulfide; magnesium selenide; germanium sulfide; germanium selenide; germanium telluride; tin sulfide; tin selenide; tin telluride; lead oxide; copper fluoride; copper chloride; copper bromide; copper iodide; copper oxide; copper selenide; nickel oxide; cobalt oxide; cobalt sulfide; triiron tetraoxide; iron sulfide; manganese oxide; molybdenum sulfide; vanadium oxide; tungsten oxide; tantalum oxide; titanium oxide; zirconium oxide; silicon nitride; germanium nitride; aluminum oxide; barium titanate; a compound of selenium, zinc, and cadmium; a compound of indium, arsenic, and phosphorus; a compound of cadmium, selenium, and sulfur; a compound of cadmium, selenium, and tellurium; a compound of indium, gallium, and arsenic; a compound of indium, gallium, and selenium; a compound of indium, selenium, and sulfur; a compound of copper, indium, and sulfur; and combinations thereof. What is called an alloyed quantum dot, whose composition is represented by a given ratio, may be used. For example, an alloyed quantum dot of cadmium, selenium, and sulfur is an effective material to obtain blue light because the emission wavelength can be changed by changing the percentages of the elements.

As the quantum dot, any of a core-type quantum dot, a core-shell quantum dot, a core-multishell quantum dot, and the like can be used. Note that when a core is covered with a shell formed of another inorganic material having a wider band gap, the influence of defects and dangling bonds existing at the surface of a nanocrystal can be reduced. Since such a structure can significantly improve the quantum efficiency of light emission, it is preferable to use a core-shell or core-multishell quantum dot. Examples of the material of a shell include zinc sulfide and zinc oxide.

Quantum dots have a high proportion of surface atoms and thus have high reactivity and easily cohere together. For this reason, it is preferable that a protective agent be attached to, or a protective group be provided at the surfaces of quantum dots. The attachment of the protective agent or the provision of the protective group can prevent cohesion and increase solubility in a solvent. It can also reduce reactivity and improve electrical stability. Examples of the protective agent (or the protective group) include polyoxyethylene alkyl ethers such as polyoxyethylene lauryl ether, polyoxyethylene stearyl ether, and polyoxyethylene oleyl ether; trialkylphosphines such as tripropylphosphine, tributylphosphine, trihexylphosphine, and trioctylphosphine; polyoxyethylene alkylphenyl ethers such as polyoxyethylene n-octylphenyl ether and polyoxyethylene n-nonylphenyl ether; tertiary amines such as tri(n-hexyl)amine, tri(n-octyl)amine, and tri(n-decyl)amine; organophosphorus compounds such as tripropylphosphine oxide, tributylphosphine oxide, trihexylphosphine oxide, trioctylphosphine oxide, and tridecylphosphine oxide; polyethylene glycol diesters such as polyethylene glycol dilaurate and polyethylene glycol distearate; organic nitrogen compounds such as nitrogen-containing aromatic compounds, e.g., pyridines, lutidines, collidines, and quinolines; aminoalkanes such as hexylamine, octylamine, decylamine, dodecylamine, tetradecylamine, hexadecylamine, and octadecylamine; dialkylsulfides such as dibutylsulfide; dialkylsulfoxides such as dimethylsulfoxide and dibutylsulfoxide; organic sulfur compounds such as sulfur-containing aromatic compounds, e.g., thiophenes; higher fatty acids such as a palmitin acid, a stearic acid, and an oleic acid; alcohols; sorbitan fatty acid esters; fatty acid modified polyesters; tertiary amine modified polyurethanes; and polyethyleneimines.

Since band gaps of quantum dots are increased as their size is decreased, the size is adjusted as appropriate so that light with a desired wavelength can be obtained. Light emission from the quantum dots is shifted to a blue color side, i.e., a high energy side, as the crystal size is decreased; thus, emission wavelengths of the quantum dots can be adjusted over a wavelength region of a spectrum of an ultraviolet region, a visible light region, and an infrared region by changing the size of quantum dots. The range of size (diameter) of quantum dots which is usually used is 0.5 nm to 20 nm, preferably 1 nm to 10 nm. The emission spectra are narrowed as the size distribution of the quantum dots gets smaller, and thus light can be obtained with high color purity. The shape of the quantum dots is not particularly limited and may be spherical shape, a rod shape, a circular shape, or the like: Quantum rods which are rod-like shape quantum dots emit directional light polarized in the c-axis direction; thus, quantum rods can be used as a light-emitting material to obtain a light-emitting element with higher external quantum efficiency.

In most EL elements, to improve emission efficiency, light-emitting materials are dispersed in host materials and the host materials need to be substances each having a singlet excitation energy or a triplet excitation energy higher than or equal to that of the light-emitting material. In the case of using a blue phosphorescent material, it is particularly difficult to develop a host material which has a triplet excitation energy higher than or equal to that of the blue phosphorescent material and which is excellent in terms of a lifetime. On the other hand, even when a light-emitting layer is composed of quantum dots and made without a host material, the quantum dots enable emission efficiency to be ensured; thus, a light-emitting element which is favorable in twins of a lifetime can be obtained. In the case where the light-emitting layer is composed of quantum dots, the quantum dots preferably have core-shell structures (including core-multishell structures).

[Alignment Film]

For the alignment films 94 and 98, a material containing polyimide or the like can be used. For example, a material containing polyimide or the like may be subjected to a rubbing process or an optical alignment process to have alignment in a predetermined direction.

[Coloring Film]

The color film 69 functions as a color filter. For the color film 69, a material transmitting light of a predetermined color (e.g., a material transmitting light of blue, green, red, yellow, or white) is used.

[Structure Body]

For the structure body 74, an organic material, an inorganic material, or an insulating material containing a composite material of an organic material and an inorganic material can be used. For the insulating material, the materials for the insulating films 32, 34, 40, 44, 48, 54, 60, 62, 68, and 72 can be used.

[Sealing Material]

For the sealing material 82, an inorganic material, an organic material, a composite material of an inorganic material and an organic material, or the like can be used. Examples of the organic material include a thermally fusible resin and a curable resin. As the sealing material 82, an adhesive including a resin material (e.g., a reactive curable adhesive, a photocurable adhesive, a thermosetting adhesive, or an anaerobic adhesive) may be used. Examples of such resin materials include an epoxy-based resin, an acrylic-based resin, a silicone-based resin, a phenol-based resin, a polyimide-based resin, an imide-based resin, a polyvinyl chloride (PVC) based resin, a polyvinyl butyral (PVB) based resin, and an ethylene vinyl acetate (EVA) based resin.

Although not illustrated in FIG. 1A to FIG. 13B, FIG. 15, and FIG. 16, the display device 500 may include components described below.

[Functional Film]

The display device 500 may include a functional film in contact with one or both of the substrates 80 and 90. As the functional film, a polarizing plate, a retardation plate, a diffusing film, an anti-reflective film, a condensing film, or the like can be used. Alternatively, an antistatic film preventing the attachment of a foreign substance, a water repellent film suppressing the attachment of stain, a hard coat film suppressing generation of a scratch in use, or the like can be used for the functional film.

[Light-Blocking Film]

The display device 500 may include a light-blocking film suppressing light transmission between adjacent pixels. Examples of the material for the light-blocking film include a metal material and an organic resin material containing a black pigment.

In this manner, two display elements can be controlled separately with the use of different transistors in the display device of one embodiment of the present invention. Accordingly, a display device having high display quality can be provided.

The structures described in this embodiment can be used in appropriate combination with any of the structures described in the other embodiments.

Embodiment 2

In this embodiment, the display device of one embodiment of the present invention which includes an input device will be described with reference to FIGS. 17A and 17B, FIG. 18, FIG. 19, and FIGS. 20A and 20B.

<2-1. Description of Input Device>

In this embodiment, a touch panel 2000 including a display device 500 and an input device will be described. In addition, an example in which a touch sensor is used as an input device will be described.

FIGS. 17A and 17B are perspective views of the touch panel 2000. Note that FIGS. 17A and 17B illustrate only main components of the touch panel 2000 for simplicity.

The touch panel 2000 includes a display device 500 and a touch sensor 2595 (see FIG. 17B). The touch panel 2000 includes a substrate 80, a substrate 90, and a substrate 2590.

The display device 500 includes a plurality of pixels over the substrate 80 and a plurality of wirings 2511 through which signals are supplied to the pixels. The plurality of wirings 2511 are led to a peripheral portion of the substrate 80, and parts of the plurality of wirings 2511 form a terminal 2519. The terminal 2519 is electrically connected to an FPC 2509(1).

The substrate 2590 includes the touch sensor 2595 and a plurality of wirings 2598 electrically connected to the touch sensor 2595. The plurality of wirings 2598 are led to a peripheral portion of the substrate 2590, and parts of the plurality of wirings 2598 form a terminal. The terminal is electrically connected to an FPC 2509(2). Note that in FIG. 17B, electrodes, wirings, and the like of the touch sensor 2595 provided on the back side of the substrate 2590 (the side facing the substrate 80) are indicated by solid lines for clarity.

As the touch sensor 2595, a capacitive touch sensor can be used, for example. Examples of the capacitive touch sensor include a surface capacitive touch sensor and a projected capacitive touch sensor.

Examples of the projected capacitive touch sensor are a self-capacitive touch sensor and a mutual capacitive touch sensor, which differ mainly in the driving method. Note that the touch sensor 2595 illustrated in FIG. 17B is an example of using a projected capacitive touch sensor. Note that a variety of sensors that can sense proximity or touch of a sensing target such as a finger can be used as the touch sensor 2595.

Note that a variety of sensors that can sense proximity or touch of a sensing target such as a finger can be used as the touch sensor 2595.

The projected capacitive touch sensor 2595 includes electrodes 2591 and electrodes 2592. The electrodes 2591 are electrically connected to any of the plurality of wirings 2598, and the electrodes 2592 are electrically connected to any of the other wirings 2598.

The electrodes 2592 each have a shape of a plurality of quadrangles arranged in one direction with one corner of a quadrangle connected to one corner of another quadrangle as illustrated in FIGS. 17A and 17B.

The electrodes 2591 each have a quadrangular shape and are arranged in a direction intersecting with the direction in which the electrodes 2592 extend.

A wiring 2594 electrically connects two electrodes 2591 between which the electrode 2592 is positioned. The intersecting area of the electrode 2592 and the wiring 2594 is preferably as small as possible. Such a structure allows a reduction in the area of a region where the electrodes are not provided, reducing variation in transmittance. As a result, variation in luminance of light passing through the touch sensor 2595 can be reduced.

Note that the shapes of the electrodes 2591 and the electrodes 2592 are not limited thereto and can be any of a variety of shapes. For example, a structure may be employed in which the plurality of electrodes 2591 are arranged so that gaps between the electrodes 2591 are reduced as much as possible, and the electrodes 2592 are spaced apart from the electrodes 2591 with an insulating layer interposed therebetween to have regions not overlapping with the electrodes 2591. In this case, it is preferable to provide, between two adjacent electrodes 2592, a dummy electrode electrically insulated from these electrodes because the area of regions having different transmittances can be reduced.

Note that as a material of the conductive films such as the electrodes 2591, the electrodes 2592, and the wirings 2598, that is, wirings and electrodes forming the touch panel, a transparent conductive film containing indium oxide, tin oxide, zinc oxide, or the like (e.g., ITO) can be given. For example, a low-resistance material is preferably used as a material that can be used as the wirings and electrodes forming the touch panel. For example, silver, copper, aluminum, a carbon nanotube, graphene, or a metal halide (such as a silver halide) may be used. Alternatively, a metal nanowire including a plurality of conductors with an extremely small width (for example, a diameter of several nanometers) may be used. Further alternatively, a net-like metal mesh with a conductor may be used. For example, an Ag nanowire, a Cu nanowire, an Al nanowire, an Ag mesh, a Cu mesh, or an Al mesh may be used. For example, in the case of using an Ag nanowire as the wirings and electrodes forming the touch panel, a visible light transmittance of 89% or more and a sheet resistance of 40 Ω/cm² or more and 100 Ω/cm² or less can be achieved. Since the above-described metal nanowire, metal mesh, carbon nanotube, graphene, and the like, which are examples of the material that can be used as the wirings and electrodes forming the touch panel, have high visible light transmittances, they may be used as electrodes of display elements (e.g., a pixel electrode or a common electrode).

<2-2. Description of Touch Sensor>

Next, the touch sensor 2595 will be described in detail with reference to FIG. 18. FIG. 18 corresponds to a cross-sectional view taken along dashed-dotted line X1-X2 in FIG. 17B.

The touch sensor 2595 includes the electrodes 2591 and the electrodes 2592 provided in a staggered arrangement on the substrate 2590, an insulating layer 2593 covering the electrodes 2591 and the electrodes 2592, and the wiring 2594 that electrically connects the adjacent electrodes 2591 to each other.

The electrodes 2591 and the electrodes 2592 are formed using a light-transmitting conductive material. As a light-transmitting conductive material, a conductive oxide such as indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added can be used. Note that a film containing graphene may be used as well. The film containing graphene can be formed, for example, by reducing a film containing graphene oxide. As a reducing method, a method with application of heat or the like can be employed.

The electrodes 2591 and the electrodes 2592 may be formed by, for example, depositing a light-transmitting conductive material on the substrate 2590 by a sputtering method and then removing an unnecessary portion by any of various patterning techniques such as photolithography.

Examples of a material for the insulating layer 2593 include a resin such as an acrylic resin or an epoxy resin, a resin having a siloxane bond, and an inorganic insulating material such as silicon oxide, silicon oxynitride, or aluminum oxide.

Openings reaching the electrodes 2591 are formed in the insulating layer 2593, and the wiring 2594 electrically connects the adjacent electrodes 2591. A light-transmitting conductive material can be favorably used as the wiring 2594 because the aperture ratio of the touch panel can be increased. Moreover, a material with higher conductivity than the conductivities of the electrodes 2591 and 2592 can be favorably used for the wiring 2594 because electric resistance can be reduced.

One electrode 2592 extends in one direction, and a plurality of electrodes 2592 are provided in the fore of stripes. The wiring 2594 intersects with the electrode 2592.

Adjacent electrodes 2591 are provided with one electrode 2592 provided therebetween. The wiring 2594 electrically connects the adjacent electrodes 2591.

Note that the plurality of electrodes 2591 are not necessarily arranged in the direction orthogonal to one electrode 2592 and may be arranged to intersect with one electrode 2592 at an angle of more than 0 degrees and less than 90 degrees.

The wiring 2598 is electrically connected to any of the electrodes 2591 and 2592. Part of the wiring 2598 functions as a terminal. For the wiring 2598, a metal material such as aluminum, gold, platinum, silver, nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or an alloy material containing any of these metal materials can be used.

Note that an insulating layer that covers the insulating layer 2593 and the wiring 2594 may be provided to protect the touch sensor 2595.

A connection layer 2599 electrically connects the wiring 2598 to the FPC 2509(2).

As the connection layer 2599, any of various anisotropic conductive films (ACF), anisotropic conductive pastes (ACP), or the like can be used.

<2-3. Description of Touch Panel>

Next, the touch panel 2000 will be described in detail with reference to FIG. 19. FIG. 19 corresponds to a cross-sectional view taken along dashed-dotted line X3-X4 in FIG. 17A.

In the touch panel 2000 illustrated in FIG. 19, the display device 500 including the pixels 10 described with reference to FIGS. 1A and 1B and the touch sensor 2595 described with reference to FIG. 18 are attached to each other.

The touch panel 2000 illustrated in FIG. 19 includes an adhesive layer 2597 and an anti-reflective layer 2569 in addition to the display device 500 including the pixels 10 described with reference to FIGS. 1A and 1B and the touch sensor 2595 described with reference to FIG. 18.

The adhesive layer 2597 is provided in contact with the wiring 2594. Note that the adhesive layer 2597 attaches the substrate 2590 to the substrate 90 so that the touch sensor 2595 overlaps with the display device 500. The adhesive layer 2597 preferably has a light-transmitting property. A heat curable resin or an ultraviolet curable resin can be used for the adhesive layer 2597. For example, an acrylic resin, a urethane-based resin, an epoxy-based resin, or a siloxane-based resin can be used.

The anti-reflective layer 2569 is positioned in a region overlapping with pixels 10. As the anti-reflective layer 2569, a circularly polarizing plate can be used, for example.

The touch panel 2000 is what is called an out-cell touch panel. Note that one embodiment of the present invention is not limited to the above structure, and the touch panel 2000 may be an in-cell touch panel or an on-cell touch panel.

<2-4. Description of Driving Method of Touch Panel>

Next, an example of a method for driving a touch panel will be described with reference to FIGS. 20A and 20B.

FIG. 20A is a block diagram illustrating the structure of a mutual capacitive touch sensor. FIG. 20A illustrates a pulse voltage output circuit 2601 and a current sensing circuit 2602. Note that in FIG. 20A, six wirings X1 to X6 represent the electrodes 2621 to which a pulse voltage is applied, and six wirings Y1 to Y6 represent the electrodes 2622 that detect changes in current. FIG. 20A also illustrates capacitors 2603 that are each formed in a region where the electrodes 2621 and 2622 overlap with each other. Note that functional replacement between the electrodes 2621 and 2622 is possible.

The pulse voltage output circuit 2601 is a circuit for sequentially applying a pulse voltage to the wirings X1 to X6. By application of a pulse voltage to the wirings X1 to X6, an electric field is generated between the electrodes 2621 and 2622 of the capacitor 2603. When the electric field between the electrodes is shielded, for example, a change occurs in the capacitor 2603 (mutual capacitance). The approach or contact of a sensing target can be sensed by utilizing this change.

The current sensing circuit 2602 is a circuit for detecting changes in current flowing through the wirings Y1 to Y6 that are caused by the change in mutual capacitance in the capacitor 2603. No change in current value is detected in the wirings Y1 to Y6 when there is no approach or contact of a sensing target, whereas a decrease in current value is detected when mutual capacitance is decreased owing to the approach or contact of a sensing target. Note that an integrator circuit or the like is used for sensing of current values.

FIG. 20B is a timing chart showing input and output waveforms in the mutual capacitive touch sensor illustrated in FIG. 20A. In FIG. 20B, sensing of a sensing target is performed in all the rows and columns in one frame period. FIG. 20B shows a period when a sensing target is not sensed (not touched) and a period when a sensing target is sensed (touched). Sensed current values of the wirings Y1 to Y6 are shown as the waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1 to X6, and the waveforms of the wirings Y1 to Y6 change in accordance with the pulse voltage. When there is no approach or contact of a sensing target, the waveforms of the wirings Y1 to Y6 change uniformly in accordance with changes in the voltages of the wirings X1 to X6. The current value is decreased at the point of approach or contact of a sensing target and accordingly the waveform of the voltage value changes.

By detecting a change in mutual capacitance in this manner, the approach or contact of a sensing target can be detected.

The structures described in this embodiment can be used in appropriate combination with any of the structures described in the other embodiments.

Embodiment 3

In this embodiment, an oxide semiconductor film of one embodiment of the present invention is described with reference to FIGS. 21A to 21C and FIGS. 22A to 22C.

<3-1. Oxide Semiconductor Film>

An oxide semiconductor film according to one embodiment of the present invention is described below.

An oxide semiconductor film preferably contains at least indium or zinc. In particular, indium and zinc are preferably contained. In addition, aluminum, gallium, yttrium, tin, or the like is preferably contained. Furthermore, one or more elements selected from boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, or the like may be contained.

Here, the case where an oxide semiconductor film is InMZnO containing indium, an element M, and zinc is considered. The element M is aluminum, gallium, yttrium, tin, or the like. Other elements that can be used as the element M include boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, and magnesium. Note that two or more of the above elements may be used in combination as the element M.

<3-2. Structure of Oxide Semiconductor Film>

An oxide semiconductor film is classified into a single crystal oxide semiconductor film and a non-single-crystal oxide semiconductor film. Examples of a non-single-crystal oxide semiconductor include a c-axis aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a nanocrystalline oxide semiconductor (nc-OS), an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor.

The CAAC-OS has c-axis alignment, its nanocrystals are connected in the a-b plane direction, and its crystal structure has distortion. Note that the distortion is a portion where the direction of a lattice arrangement changes between a region with a regular lattice arrangement and another region with a regular lattice arrangement in a region in which nanocrystals are connected.

The shape of the nanocrystal is basically a hexagon but is not always a regular hexagon and is a non-regular hexagon in many cases. A pentagonal lattice arrangement, a heptagonal lattice arrangement, and the like is included in the distortion in some cases. Note that a clear crystal grain boundary cannot be observed even in the vicinity of distortion in the CAAC-OS. That is, a lattice arrangement is distorted so that formation of a crystal grain boundary is inhibited. This is probably because the CAAC-OS can tolerate distortion owing to a low density of arrangement of oxygen atoms in an a-b plane direction, a change in interatomic bond distance by substitution of a metal element, and the like.

The CAAC-OS tends to have a layered crystal structure (also referred to as a layered structure) in which a layer containing indium and oxygen (hereinafter, In layer) and a layer containing the element Al, zinc, and oxygen (hereinafter, (M,Zn) layer) are stacked. Note that indium and the element M can be replaced with each other, and when the element M of the (M,Zn) layer is replaced with indium, the layer can also be referred to as an (In,M,Zn) layer. When indium of the In layer is replaced with the element M, the layer can also be referred to as an (In,M) layer.

In the nc-OS, a microscopic region (for example, a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic arrangement. There is no regularity of crystal orientation between different nanocrystals in the nc-OS. Thus, the orientation of the whole film is not observed. Accordingly, in some cases, the nc-OS cannot be distinguished from an a-like OS or an amorphous oxide semiconductor, depending on an analysis method.

The a-like OS has a structure intermediate between those of the nc-OS and the amorphous oxide semiconductor. The a-like OS has a void or a low-density region. That is, the a-like OS has low crystallinity as compared with the nc-OS and the CAAC-OS.

An oxide semiconductor can have various structures which show various different properties. Two or more of the amorphous oxide semiconductor, the polycrystalline oxide semiconductor, the a-like OS, the nc-OS, and the CAAC-OS may be included in an oxide semiconductor of one embodiment of the present invention.

<3-3. Atomic Ratio of Oxide Semiconductor Film>

First, preferred ranges of the atomic ratio of indium, the element M, and zinc contained in an oxide semiconductor film according to an embodiment of the present invention are described with reference to FIGS. 21A to 21C. Note that the proportion of oxygen atoms is not shown in FIGS. 21A to 21C. The terms of the atomic ratio of indium, the element M, and zinc contained in the oxide semiconductor film are denoted by [In], [M], and [Zn], respectively.

In FIGS. 21A to 21C, broken lines indicate a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):1 (where −1≦α≦1), a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):2, a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):3, a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):4, and a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):5.

In addition, dash-dotted lines indicate a line where the atomic ratio [In]:[M]:[Zn] is 5:1:β (β≧0), a line where the atomic ratio [In]:[M]:[Zn] is 2:1:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:1:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:2:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:3:β, and a line where the atomic ratio [In]:[M]:[Zn] is 1:4:β.

Dashed-double dotted lines indicate a line where the atomic ratio [In]:[M]:[Zn] is (1+γ): 2:(1−γ), where −1≦γ≦1. Furthermore, an oxide semiconductor film with the atomic ratio of [In]:[M]:[Zn]=0:2:1 or a neighborhood thereof in FIGS. 21A to 21C tends to have a spinel crystal structure.

A plurality of phases (e.g., two phases or three phases) exists in the oxide semiconductor film in some cases. For example, with an atomic ratio [In]:[M]:[Zn] that is close to 0:2:1, two phases of a spinel crystal structure and a layered crystal structure are likely to exist. In addition, with an atomic ratio [In]:[M]:[Zn] that is close to 1:0:0, two phases of a bixbyite crystal structure and a layered crystal structure are likely to exist. In the case where a plurality of phases exist in the oxide semiconductor film, a grain boundary might be Ruined between different crystal structures.

A region A in FIG. 21A represents examples of the preferred ranges of the atomic ratio of indium, the element M, and zinc contained in an oxide semiconductor film.

In addition, the oxide semiconductor film containing indium in a higher proportion can have high carrier mobility (electron mobility). This is because in an oxide semiconductor film containing indium, the element M, and zinc, the s orbital of heavy metal mainly contributes to carrier transfer, and a higher indium content in the oxide semiconductor enlarges a region where the s orbitals of indium atoms overlap. Thus, an oxide semiconductor film having a high content of indium has higher carrier mobility than an oxide semiconductor film having a low content of indium.

In contrast, when the indium content and the zinc content in an oxide semiconductor film become lower, carrier mobility becomes lower. Thus, with an atomic ratio of [In]:[M]:[Zn]=0:1:0 and the neighborhood thereof (e.g., a region C in FIG. 21C), insulation performance becomes better.

Accordingly, an oxide semiconductor film in one embodiment of the present invention preferably has an atomic ratio represented by a region A in FIG. 21A. With this atomic ratio, a layered structure with high carrier mobility and a few grain boundaries is easily obtained.

An oxide semiconductor film with an atomic ratio in the region A, particularly in a region B in FIG. 21B, is excellent because the oxide semiconductor film easily becomes a CAAC-OS and has high carrier mobility.

The CAAC-OS is an oxide semiconductor with high crystallinity. In contrast, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur because a clear grain boundary cannot be observed. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has small amounts of impurities and defects (e.g., oxygen vacancies). Thus, an oxide semiconductor including a CAAC-OS is physically stable. Therefore, the oxide semiconductor including a CAAC-OS is resistant to heat and has high reliability.

Note that the region B includes an atomic ratio of [In]:[M]:[Zn]=4:2:3 to 4.1 and the vicinity thereof. The vicinity includes an atomic ratio of [In]:[M]:[Zn]=5:3:4. Note that the region B includes an atomic ratio of [In]:[M]:[Zn]=5:1:6 and the vicinity thereof and an atomic ratio of [In]:[M]:[Zn]=5:1:7 and the vicinity thereof.

Note that the property of an oxide semiconductor film is not uniquely determined by an atomic ratio. Even with the same atomic ratio, the property of an oxide semiconductor film might be different depending on a formation condition. For example, in the case where the oxide semiconductor film is formed with a sputtering apparatus, a film having an atomic ratio deviated from the atomic ratio of a target is formed. In particular, [Zn] in the film might be smaller than [Zn] in the target depending on the substrate temperature in deposition. Thus, the illustrated regions each represent an atomic ratio with which an oxide semiconductor film tends to have specific characteristics, and boundaries of the regions A to C are not clear.

<3-4. Transistor Including Oxide Semiconductor Film>

Next, the case where the above-described oxide semiconductor film is used for a transistor will be described.

Note that when the oxide semiconductor film is used for a transistor, carrier scattering or the like at a grain boundary can be reduced; thus, the transistor can have high field-effect mobility. In addition, the transistor can have high reliability.

An oxide semiconductor film with low carrier density is preferably used for the transistor. In order to reduce the carrier density of the oxide semiconductor film, the impurity concentration in the oxide semiconductor film is reduced so that the density of defect states can be reduced. In this specification and the like, a state with a low impurity concentration and a low density of defect states is referred to as a highly purified intrinsic or substantially highly purified intrinsic state. For example, an oxide semiconductor film whose carrier density is lower than 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, or further preferably lower than 1×10¹⁰/cm³, and greater than or equal to 1×10⁻⁹/cm³ is used as the oxide semiconductor film.

A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly has few carrier traps in some cases.

Charges trapped by the trap states in the oxide semiconductor film take a long time to be released and may behave like fixed charges. Thus, the transistor whose channel region is formed in the oxide semiconductor film having a high density of trap states has unstable electrical characteristics in some cases.

To obtain stable electrical characteristics of the transistor, it is effective to reduce the concentration of impurities in the oxide semiconductor film. In order to reduce the concentration of impurities in the oxide semiconductor film, the concentration of impurities in a film that is adjacent to the oxide semiconductor film is preferably reduced. As examples of the impurities, hydrogen, nitrogen, alkali metal, alkaline earth metal, iron, nickel, silicon, and the like are given.

<3-5. Impurities in Oxide Semiconductor Film>

Here, the influence of impurities in the oxide semiconductor film will be described.

When silicon or carbon that is one of Group 14 elements is contained in the oxide semiconductor film, defect states are formed in the oxide semiconductor film. Thus, the concentration of silicon or carbon (measured by secondary ion mass spectrometry (SIMS)) in the oxide semiconductor film and around an interface with the oxide semiconductor film is set lower than or equal to 2×10¹⁸ atoms/cm³, and preferably lower than or equal to 2×10¹⁷ atoms/cm³.

When the oxide semiconductor film contains alkali metal or alkaline earth metal, defect states are formed and carriers are generated, in some cases. Thus, a transistor including an oxide semiconductor film which contains alkali metal or alkaline earth metal is likely to be normally on. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal in the oxide semiconductor film. Specifically, the concentration of alkali metal or alkaline earth metal of the oxide semiconductor film, which is measured by SIMS, is lower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁶ atoms/cm³.

When the oxide semiconductor film contains nitrogen, the oxide semiconductor film easily becomes n-type by generation of electrons serving as carriers and an increase of carrier density. Thus, a transistor whose semiconductor includes an oxide semiconductor film that contains nitrogen is likely to be normally-on. For this reason, nitrogen in the oxide semiconductor film is preferably reduced as much as possible; for example, the concentration of nitrogen in the oxide semiconductor film measured by SIMS is set to be lower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³, and still further preferably lower than or equal to 5×10¹⁷ atoms/cm³.

Hydrogen contained in an oxide semiconductor film reacts with oxygen bonded to a metal atom to be water, and thus causes an oxygen vacancy, in some cases. Entry of hydrogen into the oxygen vacancy generates an electron serving as a carrier in some cases. Furthermore, in some cases, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier. Thus, a transistor including an oxide semiconductor film that contains hydrogen is likely to be normally on. Accordingly, it is preferable that hydrogen in the oxide semiconductor film be reduced as much as possible. Specifically, the hydrogen concentration in the oxide semiconductor film measured by SIMS is set lower than 1×10²⁰ atoms/cm³, preferably lower than 1×10¹⁹ atoms/cm³, further preferably lower than 5×10¹⁸ atoms/cm³, and still further preferably lower than 1×10¹⁸ atoms/cm³.

When an oxide semiconductor film with sufficiently reduced impurity concentration is used for a channel region in a transistor, the transistor can have stable electrical characteristics.

<3-6. Band Diagram>

Next, the case where the oxide semiconductor film described above has a two-layer structure or three-layer structure is described.

A band diagram of a stacked-layer structure of an oxide semiconductor film S1, an oxide semiconductor film S2, and an oxide semiconductor film S3 and insulating films that are in contact with the stacked-layer structure, a band diagram of a stacked-layer structure of the oxide semiconductor films S2 and S3 and insulating films that are in contact with the stacked-layer structure, and a band diagram of a stacked-layer structure of the oxide semiconductor films S1 and S2 and insulating films that are in contact with the stacked-layer structure are described with reference to FIGS. 22A to 22C.

FIG. 22A is an example of a band diagram of a layered structure including an insulating film I1, the oxide semiconductor film S1, the oxide semiconductor film S2, the oxide semiconductor film S3, and an insulating film I2 in a thickness direction. FIG. 22B is an example of a band diagram of a layered structure including the insulating film I1, the oxide semiconductor film S2, the oxide semiconductor film S3, and the insulating film 12 in a thickness direction. FIG. 22C is an example of a band diagram of a layered structure including the insulating film I1, the oxide semiconductor film S1, the oxide semiconductor film S2, and the insulating film 12 in a thickness direction. Note that for easy understanding, the band diagrams show the conduction band minimum (Ec) of each of the insulating film I1, the oxide semiconductor film S1, the oxide semiconductor film S2, the oxide semiconductor film S3, and the insulating film I2.

The conduction band minimum of each of the oxide semiconductor films S1 and S3 is closer to the vacuum level than that of the oxide semiconductor film S2.

Typically, a difference between the conduction band minimum of the oxide semiconductor film S2 and the conduction band minimum of each of the oxide semiconductor films S1 and S3 is preferably greater than or equal to 0.15 eV or greater than or equal to 0.5 eV, and less than or equal to 2 eV or less than or equal to 1 eV. That is, it is preferable that the difference between the electron affinity of each of the oxide semiconductor films S1 and S3 and the electron affinity of the oxide semiconductor film S2 be greater than or equal to 0.15 eV or greater than or equal to 0.5 eV, and less than or equal to 2 eV or less than or equal to 1 eV.

As illustrated in FIGS. 22A to 22C, the conduction band minimum of each of the oxide semiconductor films S1 to S3 is gradually varied. In other words, the energy level of the conduction band minimum is continuously varied or continuously connected. In order to obtain such a band diagram, the density of defect states in a mixed layer formed at an interface between the oxide semiconductor films S1 and S2 or an interface between the oxide semiconductor films S2 and S3 is preferably made low.

Specifically, when the oxide semiconductor films S1 and S2 or the oxide semiconductor films S2 and S3 contain the same element (as a main component) in addition to oxygen, a mixed layer with a low density of defect states can be formed. For example, in the case where the oxide semiconductor film S2 is an In—Ga—Zn oxide semiconductor film, it is preferable to use an In—Ga—Zn oxide semiconductor film, a Ga—Zn oxide semiconductor film, gallium oxide, or the like as each of the oxide semiconductor films S1 and S3.

At this time, the oxide semiconductor film S2 serves as a main carrier path. Since the density of defect states at the interface between the oxide semiconductor films S1 and S2 and the interface between the oxide semiconductor films S2 and S3 can be made low, the influence of interface scattering on carrier conduction is small, and high on-state current can be obtained.

When an electron is trapped in a trap state, the trapped electron behaves like fixed charge; thus, the threshold voltage of the transistor is shifted in a positive direction. The oxide semiconductor films S1 and S3 can make the trap state apart from the oxide semiconductor film S2. This structure can prevent the positive shift of the threshold voltage of the transistor.

A material whose conductivity is sufficiently lower than that of the oxide semiconductor film S2 is used for the oxide semiconductor films S1 and S3. Accordingly, the oxide semiconductor film S2, the interface between the oxide semiconductor films S1 and S2, and the interface between the oxide semiconductor films S2 and S3 mainly function as a channel region. For example, an oxide semiconductor film with high insulation performance and the atomic ratio represented by the region C in FIG. 21C can be used as the oxide semiconductor films S1 and S3. Note that the region C illustrated in FIG. 21C represents atomic ratios [In]:[M]:[Zn] of 0:1:0, 1:3:2, and 1:3:4 and the vicinities thereof.

In the case where an oxide semiconductor film with the atomic ratio represented by the region A is used as the oxide semiconductor film S2, it is particularly preferable to use an oxide with an atomic ratio where [M]/[In] is greater than or equal to 1, preferably greater than or equal to 2 as each of the oxide semiconductor films S1 and S3. Moreover, the oxide semiconductor film S3 is preferably an oxide semiconductor film having [M]/([Zn]+[In]) of 1 or greater to obtain sufficiently high insulation performance.

Note that the structures described in this embodiment can be used in combination with any of the structures described in the other embodiments as appropriate.

Embodiment 4

In this embodiment, a display module and electronic devices that include the display device of one embodiment of the present invention will be described with reference to FIG. 23, FIGS. 24A to 24E, and FIGS. 25A to 25E.

<4-1. Display Module>

In a display module 8000 illustrated in FIG. 23, a touch panel 8004 connected to an FPC 8003, a display panel 8006 connected to an FPC 8005, a frame 8009, a printed circuit board 8010, and a battery 8011 are provided between an upper cover 8001 and a lower cover 8002.

The display device of one embodiment of the present invention can be used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002 can be changed as appropriate in accordance with the sizes of the touch panel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitive touch panel and may overlap with the display panel 8006. Alternatively, a counter substrate (sealing substrate) of the display panel 8006 can have a touch panel function. Alternatively, a photosensor may be provided in each pixel of the display panel 8006 so as to function as an optical touch panel.

The frame 8009 protects the display panel 8006 and functions as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed circuit board 8010. The frame 8009 may function as a radiator plate.

The printed circuit board 8010 is provided with a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or a power source using the battery 8011 provided separately may be used. The battery 8011 can be omitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet.

<4-2. Electronic Device>

FIGS. 24A to 24E and FIGS. 25A to 25E illustrate electronic devices. These electronic devices can include a housing 9000, a display portion 9001, a camera 9002, a speaker 9003, an operation key 9005 (including a power switch or an operation switch), a connection terminal 9006, a sensor 9007 (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone 9008, and the like.

The electronic devices illustrated in FIGS. 24A to 24E and FIGS. 25A to 25E can have a variety of functions, for example, a function of displaying a variety of data (a still image, a moving image, a text image, and the like) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling a process with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, a function of reading a program or data stored in a memory medium and displaying the program or data on the display portion, and the like. Note that functions of the electronic devices illustrated in FIGS. 24A to 24E and FIGS. 25A to 25E are not limited thereto, and the electronic devices may have other functions.

The electronic devices illustrated in FIGS. 24A to 24E and FIGS. 25A to 25E are described in detail below.

FIG. 24A is a perspective view illustrating a television device 9100. The television device 9100 can include the display portion 9001 having a large screen size of, for example, 50 inches or more, 80 inches or more, or 100 inches or more.

FIG. 24B, FIG. 24C, FIG. 24D, and FIG. 24E are perspective views illustrating a portable information terminal 9101, a portable information terminal 9102, a portable information terminal 9103, and a portable information terminal 9104, respectively.

The portable information terminal 9101 illustrated in FIG. 24B has, for example, one or more of a function of a telephone set, a notebook, and an information browsing system. Specifically, the portable information terminal 9101 can be used as a smartphone. Although not illustrated, the speaker 9003, the connection terminal 9006, the sensor 9007, and the like may be provided in the portable information terminal 9101. The portable information terminal 9101 can display characters and image information on its plurality of surfaces. For example, three operation buttons 9050 (also referred to as operation icons or simply icons) can be displayed on one surface of the display portion 9001. Furthermore, information 9051 indicated by dashed rectangles can be displayed on another surface (for example, a side surface) of the display portion 9001. Examples of the information 9051 include notification from a social networking service (SNS), display indicating reception of an e-mail or an incoming call, the title of the e-mail, the SNS, or the like, the sender of the e-mail, the SNS, or the like, the date, the time, remaining battery, and the strength of a received signal. Alternatively, the operation buttons 9050 or the like may be displayed in place of the information 9051. The display portion 9001 of the portable information terminal 9101 partly has a curved surface.

The portable information terminal 9102 illustrated in FIG. 24C has a function of displaying information, for example, on three or more sides of the display portion 9001. Here, information 9052, information 9053, and information 9054 are displayed on different sides. For example, a user of the portable information terminal 9102 can see the display (here, the information 9053) with the portable information terminal 9102 put in a breast pocket of his/her clothes. Specifically, a caller's phone number, name, or the like of an incoming call is displayed in a position that can be seen from above the portable information terminal 9102. Thus, the user can see the display without taking out the portable information terminal 9102 from the pocket and decide whether to answer the call. The display portion 9001 of the portable information terminal 9102 partly has a curved surface.

Unlike in the portable information terminals 9101 and 9102 described above, the display portion 9001 does not have a curved surface in the portable information terminal 9103 illustrated in FIG. 24D.

The display portion 9001 of the portable information terminal 9104 illustrated in FIG. 24E is curved. As illustrated in FIG. 24E, it is preferable that the portable information terminal 9104 be provided with a camera 9002 to have a function of taking a still image, a function of taking a moving image, a function of storing the taken image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying the taken image on the display portion 9001, or the like.

FIG. 25A is a perspective view of a watch-type portable information terminal 9200. FIG. 25B is a perspective view of a watch-type portable information terminal 9201.

The portable information terminal 9200 illustrated in FIG. 25A is capable of executing a variety of applications such as mobile phone calls, e-mailing, viewing and editing texts, music reproduction, Internet communication, and computer games. The display surface of the display portion 9001 is bent, and images can be displayed on the bent display surface. The portable information terminal 9200 can employ near field communication that is a communication method based on an existing communication standard. In that case, for example, mutual communication between the portable information terminal 9200 and a headset capable of wireless communication can be performed, and thus hands-free calling is possible. Moreover, the portable information terminal 9200 includes the connection terminal 9006, and data can be directly transmitted to and received from another information terminal via a connector. Charging through the connection terminal 9006 is possible. Note that the charging operation may be performed by wireless power feeding without using the connection terminal 9006.

Unlike in the portable information terminal 9200 illustrated in FIG. 25A, the display surface of the display portion 9001 is not curved in the portable information terminal 9201 illustrated in FIG. 25B. Furthermore, the external state of the display portion of the portable information terminal 9201 is a non-rectangular shape (a circular shape in FIG. 25B).

FIGS. 25C, 25D, and 25E are perspective views of a foldable portable information terminal 9202. FIG. 25C is a perspective view illustrating the portable information terminal 9202 that is opened. FIG. 25D is a perspective view illustrating the portable information terminal 9202 that is being opened or being folded. FIG. 25E is a perspective view illustrating the portable information terminal 9202 that is folded.

The folded portable information terminal 9202 is highly portable, and the opened portable information terminal 9202 is highly browsable due to a seamless large display region. The display portion 9001 of the portable information terminal 9202 is supported by three housings 9000 joined together by hinges 9055. By folding the portable information terminal 9202 at a connection portion between two housings 9000 with the hinges 9055, the portable information terminal 9202 can be reversibly changed in shape from opened to folded. For example, the portable information terminal 9202 can be bent with a radius of curvature of greater than or equal to 1 mm and less than or equal to 150 mm.

The display device which is one embodiment of the present invention can be preferably used for the display portion 9001.

Electronic devices described in this embodiment are characterized by having a display portion for displaying some sort of information. Note that the semiconductor device of one embodiment of the present invention can also be used for an electronic appliance that does not have a display portion.

The structure described in this embodiment can be used in appropriate combination with any of the structures described in the other embodiments.

Example

A display device including the pixels 10 with the structure illustrated in FIGS. 1A and 1B in Embodiment 1 was fabricated, and the display results of the display device were evaluated.

Tables 1 and 2 show specifications of the display device fabricated in this example.

TABLE 1 Specifications Panel size 4.3 inches Number of effective pixels 1080 (H) × 1920 (V) Pixel size 49.5 μm (H) × 49.5 μm (V) External panel size 76.0 mm (H) × 130.0 mm (V) Display region 53.46 mm (H) × 95.04 mm (V) Resolution 513 ppi LCD Reflective twisted ECB mode Aperture ratio 56.7% Drive frequency 60 Hz Video signal format Analog line sequential Gate driver Built-in Source driver Driver IC

TABLE 2 Specifications Panel size 4.3 inches Number of effective pixels 1080 (H) × 1920 (V) Pixel size 49.5 μm (H) × 49.5 μm (V) External panel size 76.0 mm (H) × 130.0 mm (V) Display region 53.46 mm (H) × 95.04 mm (V) Resolution 513 ppi OLED Bottom emission (Tandem white + CF) Aperture ratio R: 25.6%, G: 25.2%, B: 25.1%, and W: 5.7%, Average: 20.4% Drive frequency 60 Hz Video signal format Analog line sequential Gate driver Built-in Source driver Driver IC

Table 1 shows the specifications of the display device including the LCD elements. Table 2 shows the specifications of the display device including the OLED elements.

<Results of Display on Display Device>

Next, results of display on the display device fabricated in this example is shown in FIGS. 26A and 26B. FIG. 26A shows display results of a display mode (color display) using an OLED element that is a light-emitting element. FIG. 26B shows display results of a display mode (monochrome display) using a reflective LCD element.

As shown in FIGS. 26A and 26B, it was confirmed that the display device of one embodiment of the present invention had a favorable display quality.

Note that the structures described in this example can be used in combination with any of the structures described in the other embodiments as appropriate.

This application is based on Japanese Patent Application serial no. 2016-089154 filed with Japan Patent Office on Apr. 27, 2016, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A display device comprising: a first display element; a second display element; a first transistor; a second transistor; and a third transistor, wherein the first display element comprises a liquid crystal layer, wherein the second display element comprises a light-emitting layer, wherein the first transistor is configured to select the first display element, wherein the second transistor is configured to select the second display element, wherein the third transistor is configured to control driving of the second display element, wherein the first transistor and the second transistor are located on a same surface, and wherein the third transistor is located over the first transistor and the second transistor and comprises one of a source electrode and a drain electrode of the second transistor as a gate electrode.
 2. The display device according to claim 1, wherein the third transistor comprises a plurality of gate electrodes.
 3. The display device according to claim 1, wherein one or more of the first transistor, the second transistor, and the third transistor comprise an oxide semiconductor in a semiconductor layer.
 4. The display device according to claim 1, wherein the light-emitting layer is configured to emit light toward the liquid crystal layer side.
 5. A display module comprising: the display device according to the claim 1; and a touch sensor.
 6. An electronic device comprising: the display device according to claim 1; and an operation key or a battery.
 7. An electronic device comprising: the display module according to claim 5; and an operation key or a battery.
 8. A display device comprising: a first display element; a second display element; a first transistor; a second transistor; a third transistor; and a capacitor, wherein the first display element comprises a first pixel electrode and a liquid crystal layer, wherein the second display element comprises a second pixel electrode and a light-emitting layer, wherein the first transistor is electrically connected to the first pixel electrode, wherein the second transistor is electrically connected to the second pixel electrode, wherein the third transistor is electrically connected to the second display element, wherein the capacitor comprises a pair of electrodes, wherein one of the pair of electrodes comprises a capacitor electrode, wherein the other of the pair of electrodes comprises the first pixel electrode, wherein the first transistor and the second transistor are located on a same surface, and wherein the third transistor is located over the first transistor and the second transistor and comprises one of a source electrode and a drain electrode of the second transistor as a gate electrode.
 9. The display device according to claim 8, wherein the capacitor electrode is positioned below one or both of the first transistor and the second transistor.
 10. The display device according to claim 8, wherein the first pixel electrode is configured to reflect light, and wherein the second pixel electrode is configured to transmit light.
 11. The display device according to claim 8, wherein the first pixel electrode comprises one or both of silver and aluminum, and wherein the second pixel electrode comprises one or more of indium, zinc, tin, and silicon.
 12. The display device according to claim 8, wherein the third transistor comprises a plurality of gate electrodes.
 13. The display device according to claim 8, wherein one or more of the first transistor, the second transistor, and the third transistor comprise an oxide semiconductor in a semiconductor layer.
 14. The display device according to claim 8, wherein the light-emitting layer is configured to emit light toward the liquid crystal layer side.
 15. A display module comprising: the display device according to claim 8; and a touch sensor.
 16. An electronic device comprising: the display device according to claim 8; and an operation key or a battery.
 17. An electronic device comprising: the display module according to claim 15; and an operation key or a battery. 